On May 5th, 2022, the FDA announced the availability of a draft guidance for industry entitled Clinical Pharmacology Considerations for Human Radiolabeled Mass Balance Studies. This draft guidance describes the FDA’s recommendations regarding clinical pharmacology considerations for conducting human radiolabeled mass balance studies of investigational drugs, including:

  • Deciding whether and when to conduct the study,
  • Designing the study
  • Reporting results.


A human radiolabeled mass balance study is the single most direct method to obtain quantitative and comprehensive information on the absorption, distribution, metabolism, and excretion of the drug in the human body.


The mass balance study can provide information to determine the overall pathways of metabolism and excretion of an investigational drug, identify circulating metabolites, and determine the abundance of metabolites relative to the parent or total drug-related exposure.
Furthermore, results from mass balance studies provide information on which metabolites should be structurally characterized or should undergo nonclinical safety assessment or drug-drug interaction (DDI) evaluation.

Data from such studies can also help determine whether renal or hepatic impairment studies or certain DDI studies are recommended for the investigational drug.

FDA Draft Guidance’s:

The FDA encourages Industry and individuals to comment on the draft guidance. The comments to the draft do make a difference and can impact the outcomes of FDA regulatory policy.

You may submit your comments regarding the draft guidance to the docket (FDA-2022-D-0113) available at https://www.regulations.gov up to 90 days following publication in the FEDERAL REGISTER. This draft guidance, when finalized, will represent the current thinking of the FDA on this topic. It does not establish any rights for any person and is not binding on FDA or the public.

Learn more Clinical Pharmacology

The “Clinical Pharmacology Considerations for Human Radiolabeled Mass Balance Studies” guidance is available at: https://go.usa.gov/xupUp. Please refer to the draft guidance for more details.

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